This invention is directed to a line pressure control system for a continuously variable transmission (CVT). Such a transmission includes fluid-actuated primary and secondary pulleys intercoupled by a flexible chain or belt, the effective diameters of the pulleys being varied simultaneously but in opposite directions to effect a smooth, continuous change in the drive ratio between an input driving shaft and an output driven shaft. The primary pulley is controlled by fluid under pressure to regulate the effective drive ratio of the transmission, while the secondary pulley is supplied with fluid at a pressure sufficient to prevent slippage of the belt in order to transfer torque from the primary pulley to the secondary pulley. A fluid-actuated clutch is generally employed to transfer drive torque from the transmission (namely, from the secondary pulley) to an associated driveline. The line pressure or secondary pressure is the highest hydraulic pressure in the CVT and is that which is applied to the secondary pulley to ensure an adequate clamping force and tension on the belt so that it will not slip.
Various systems have been developed for controlling the different hydraulic pressures required to operate a CVT, such as those systems described in U.S. Pat. Nos. 4,718,308, 4,648,496 and 4,982,822, which are incorporated herein by reference.
In the typical CVT control system, the hydraulic pressure acting on the primary sheave during a downshift is reduced to allow the primary sheave halves to open up and reduce the effective diameter of the primary sheave. It has been found that when downshifting under some conditions there is insufficient clamping force and slippage occurs between the primary drive sheave and the drive chain or belt. Such slippage can result in damage to the sheaves and chain or belt.